1. Field of the Invention
The invention relates to a cooling method for an illumination device, and more particularly to a method for adjusting the rotational rate of a cooling fan of an illumination device.
2. Description of the Related Art
Illumination devices are typically employed in a variety of apparatuses, such as projection devices, rear-projection televisions, household illumination devices, commercial neon illumination devices, and high altitude illumination devices. Because illumination devices become hot when used, heat-dissipating devices, such as cooling fans, are typically required for cooling.
Current fan control methods hold the fan at a fixed rotational rate. In some methods, several references, such as pulses, are collected from the fan to maintain a stable rate of rotation. In some methods, the cooling fans are set to operate at a fixed rate of rotation for cooling illumination devices with power greater than a specific wattage, such as lamps with power greater than 100 watts, are used.
An arc gap between a cathode and an anode of an illumination device increases with time used. Lamp voltage is generated between the cathode and the anode in direct proportion to the arc gap. Thus, lamp voltage rises gradually as increased time of use used. Because the lamp voltage rises, the temperature of the illumination device increases slightly, and the illumination device does not remain at the same temperature from the initial usage stage to the later usage stage. In other words, an illumination device in use for a long time is hotter and less bright.
Referring to FIG. 1, a lamp is illustrated as an example of an illumination device. A lamp 10 comprises a cathode 12, an anode 14, and a cooling fan 16. An arc is formed to emit light by the discharge between the cathode 12 and the anode 14, and an arc gap 18 between the cathode 12 and the anode 14 increases with increased usage time. The lamp voltage which is the voltage difference between the cathode 12 and the anode 14 gradually increases with the increased usage time of the lamp 10. The greater the lamp voltage becomes, the hotter the lamp 10 will be.
Current control methods, however, do not consider lamp voltage or temperature variation in an illumination device. Adjusting the rotational rate of a cooling fan based on the variation of the lamp voltage in an illumination device is a more efficient and necessary method to cooling the illumination device. Thus, the optimum working temperature of the illumination device will be maintained.